if_vlan: add the ALTQ support to if_vlan.

Inspired by the iflib implementation, allow ALTQ to be used with if_vlan
interfaces.

Reviewed by: donner
Obtained from: pfsense
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D31647